1. Field of the Invention
The present invention relates generally to a process for the catalyst initiated reaction of polyol and isocyanate compounds, for the manufacture of polyurethane prepolymer adhesive under room temperature conditions and in convenient quantities for application in industry.
2. Prior Art
Previously polyurethane prepolymerized adhesive has been manufactured by the general method of mixing suitable polyol, isocyanate, and additive compounds, such as fire retardants and plasticizers, in a large industrial capacity reaction kettle, and maintaining the mixture in a sealed reaction kettle within a high temperature range until the reaction has gone to completion, generally 2 to 4 hours. The temperature range will depend upon the polyol and isocyanate reacted, but temperatures frequently must be maintained between 175.degree. F. and 185.degree. F. Heating systems are required to initiate the reaction, but since the reaction is highly exothermic, cooling systems are also required to prevent undesirable side reactions. Large, expensive reaction kettles are required in order for the process to generate sufficient heat to sustain the reaction once it has been initiated. The demands of a large reaction kettle, heating and cooling controls, heaters, and stirrers call for a large financial investment in the reaction apparatus. Supervision and control of the process further require considerable operator attention so that the manpower requirements for the industrial process are high.
Once the prepolymerization reaction of the polyurethane adhesive has subsided, the product must be transferred to suitable drums, cans, pails or other containers for storage and use. During the prepolymerization reaction and during transfer and storage, good urethane polymer practices of blanketing the materials with dry air or nitrogen must be observed, and all containers and utensils require scrupulous cleaning and drying.
The polyurethane adhesive being a one-component, moisture cured system, is highly reactive to moisture. Containers must be sealed to protect the contents from atmospheric moisture. Once a container is opened for use, the entire contents must be protected from moisture with a dry nitrogen blanket, or by means of an air-drying train. The air-drying train may use such well-known drying agents as anhydrous calcium sulfate, anhydrous silica gel, andhydrous magnesium perchlorate, molecular sieves, etc. When small quantities are to be used, it may be more convenient to prepare the adhesive in containers sized to hold the desired quantity. The adhesive can then be used directly from the small containers without substantial waste.
This technology is well known in the prior art, and the general principles involved are explained in "The Development and Use of Polyurethane Products", by E. N. Doyle, McGraw-Hill Book Company, 1971 and in "Organic Polymer Chemistry", by K. J. Saunders, Chapman and Hall, 1973.
Catalysts for hastening the reaction of polyols and isocyanates are known, and are either generally organometallic or diamine catalysts, although other materials such as alkaline compounds have also served as catalysts to some degree. However, it is believed that because of the extremely close control of the reaction conditions for adhesive applications no successful catalyst initiated polyurethane prepolymer adhesive of the type discussed herein has been commercially available. Also, the ratio of NCO to OH groups must be controlled to obtain the desired bonding and handling properties for the adhesive.
The present invention provides a simple method for producing catalyst initiated polyurethane prepolymer adhesive under convenient room temperature conditions and in convenient volume containers for storage and application in industry. The proper sequence of adding and mixing the required components and the proper amount and type of catalyst are important aspects of the present invention for obtaining an adhesive system with strong bonding characteristics and good "pot life". To produce an adhesive with good bonding properties and physical characteristics the ratio of NCO to OH groups must be controlled and the mixing container must be clean and free of moisture and other contamination.